Transfer switches assemblies have been used for years in locations where it is necessary to use a secondary power source so that an electrical load may be automatically transferred to this source when a primary power source fails.
An important requirement of any transfer switch assembly is to prevent the simultaneous connection of the electrical load to the primary and to the secondary power source. To prevent this from occurring, the power source connected to the electrical load must always be disconnected before it is reconnected to the other power source. This requirement often leads to complex arrangements of mechanical components to move the actuating members of the switching mechanisms connecting the electrical load to the power sources.
In many known transfer switches assemblies, an electrical motor is used to move the actuating members of the switching mechanism from an opened to a closed position, or vice-versa. These motors often need gears and speed reducing devices to slow down the transitions. Furthermore, if an electrical motor is used, a motion limiting system is necessary to prevent overshooting.
Other transfer switches assemblies use a solenoid to move the actuating members of the switching mechanism from an opened to a closed position, or vice-versa. The major drawback of using a solenoid is the high speed of transition between the disconnection of the electrical load from one power source and the subsequent connection of the electrical load to the other power source. Indeed, repeated fast transitions may prematurely wear the moving parts of the transfer switch assembly, especially the switching devices. Furthermore, if the electrical load is of the type that regenerate electricity immediately subsequent to disconnection from one power source, the regenerated electricity may be discharged in the other power source, possibly causing problems.
Another drawback of the known transfer switches assemblies is their size. In many applications, transfer switches are installed in enclosures having small volumes, for example in pump controllers where a transfer switch assembly selects, from the electric network or an emergency electric generator, the power source supplying electricity to the pump.